The invention pertains to a composition which when intimately mixed with water forms an aqueous treatment fluid suitable for use as a well completion or spacer fluid in the drilling of oil and gas wells or other boreholes in subterranean formations. It also relates to a method of using such an aqueous treatment fluid as a spacer composition, particularly in the context of cementing casing in a wellbore.
Rotary drilling techniques are commonly used for drilling wells in the earth through subterranean formations of sandstone, shale, limestone, etc. In such rotary drilling, a drilling fluid or "drilling mud" is circulated between the surface of the earth and the bottom of the well. Drilling muds which are commonly used include water-based muds comprising both clay and polymer containing muds, oil-based muds and emulsions. Drilling muds serve many useful purposes including the removal of earth and rock cuttings from the well, control of formation pressures, and cooling and lubrication of the drill bit which is used in drilling the well. However, there are also certain detrimental characteristics associated with drilling muds. Among the problems associated with drilling muds is that drilling muds tend to flow from the well into exposed permeable formations with the result that mud solids are filtered out on the wall of the well and a filter cake is formed. Even thin filter cakes are detrimental in the completion of wells because they sometimes interfere with the formation of a good cement bond between the wall of the wellbore and the casing positioned in this wellbore. Also, drilling muds frequently contain components which are incompatible with a fluid which one may desire to inject into a well containing such mud. For example, it has long been recognized that if certain cement slurries containing free polyvalent metal cations, especially calcium, are brought into contact with muds containing clay or certain polymers, a very viscous and detrimental plug can form in the vicinity of the mud-cement interface. The problems created by such a highly viscous mud-cement interface are well known in the well cementing art. Another example of mud-cement incompatibility is that lignins, which are frequently used as dispersants in high density muds, can cause excessive retardation in cements if cement becomes comingled with the mud.
Consequently, various techniques have been devised for the removal of drilling muds from a borehole, particularly in the context of injecting a fluid into the borehole which is incompatible with the mud, more specifically in the context of cementing. A common technique is to employ a "spacer" or a "chemical wash". Although it is not always clear in the literature whether a fluid is a spacer or a chemical wash, a spacer is generally characterized as a thickened composition which functions primarily as a fluid piston in displacing the mud. Spacers frequently contain appreciable quantities of weighting agents to impart a desired density to the spacer fluid. Chemical washes, on the other hand, are generally thin fluids which are effective principally as the result of turbulence, dilution, and surfactant action on the mud and mud filter cake. Chemical washes often contain solids to act as an abrasive but the solids content is significantly lower than in spacers because the washes are generally too thin to have good solids carrying capacity.
The art of preparing and using spacers and chemical washes is discussed in detail in U.S. Pat. No. 4,083,407, the teachings of which are hereby incorporated by reference. Of particular interest is U.S. Pat. No. 3,291,211, Savins et al. which describes a process for removing mudcake from the wall of a borehole and then cementing casing in such a wellbore by using a fluid said to have "viscoelastic" properties as a spacer to displace a mudcake from the annulus. The viscoelastic fluid is said to be prepared from an oil-miscible solvent for use with an oil-based drilling mud or from water or water-miscible solvent in the case of use with a water-based drilling mud. Viscoelastic properties are imparted to the spacer fluids by the addition of various solutes to the respective solvents. Among the solutes mentioned are high molecular weight cellulose derivatives such as carboxymethylcellulose (CMC) and hydroxyethylcellulose (HEC), polyethylene oxides, sulfonated polystyrenes, polyacrylamides and partially hydrolyzed polyacrylamides, natural gums, fatty acid soaps, etc. Savins prefers that his viscoelastic fluids be such that a normal stress of at least 2,000 dynes per square centimeter is developed in a rotating viscometer.
In U.S. Pat. No. 4,141,843, Watson discloses a nondamaging spacer fluid comprising weighting agents dispersed in water, a polymer viscosifier and several other components. The spacer is said to be stable over a temperature range of about 32.degree.-300.degree. F. for extended periods of time. The fluids described by Watson employ as preferred viscosifiers water soluble polysaccharides, especially hydroxyalkylcelluloses having two to three carbon atoms in the hydroxyalkyl groups. The spacer fluids prepared by Watson are said to be non-Newtonian in character, i.e. they have yield points which are not equal to zero.
In a publication entitled "Applied Engineered Cementing" (1969) by Byron Jackson, Inc., pages 54-55, a "cementing preflush", or spacer as the term is used herein, designated Mud-Sweep is described as a high viscosity, aqueous solution having a density which can be adjusted from 8.6 to as high as 18 pounds per gallon. That spacer is said to operate on a "viscous sweep" principle to accomplish maximum mud removal regardless of the flow rate. It is said not to be necessary to attain turbulent flow to achieve excellent mud removal. The composition of the spacer is not disclosed, however it is stated to be highly viscous yet thixotropic, to contain de-oiling chemicals that tend to preferentially water wet the casing and wellbore and to also contain additives that "flocculate clay-base drilling muds, forming a viscous interface that aids in sweeping mud from the annulus". Obviously, that spacer is incompatible with many drilling fluids.
In U.S. Pat. No. 4,083,407 CMC and HEC are employed by Griffin et al. as fluid loss control additives. The spacer itself is thickened with a polyvalent metal silicate.